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The connection between insomnia and an increased risk for heart attack isn't clear, but sleep problems might have an effect on blood pressure or inflammation, which can both be risk factors for a heart attack.
"As insomnia symptoms are common and fairly easy to treat, it is important that people are aware of this connection between insomnia and heart attack, and talk to their doctors if they have sleep problems," said lead researcher Dr. Lars Erik Laugsand, an internist from the Norwegian University of Science and Technology in Trondheim.
The finding remains an association, and cause-and-effect has not been proven. Further studies are needed to confirm the findings and to uncover the possible mechanisms behind the association, Laugsand said.
The report was published in the Oct. 24 online edition of Circulation.
For the study, Laugsand's team collected data on almost 53,000 men and women who took part in a national health survey in 1995-97 and answered questions about their sleep habits. In addition, the researchers identified almost 2,400 people who had a first heart attack over the following 11 years.
The researchers found that people who had trouble falling asleep almost every day had a 45% increased risk of a heart attack, compared with those who had no problem going to sleep.
In addition, people who had trouble staying asleep had a 30% higher risk of a heart attack, compared with those who had no trouble staying asleep, the authors noted. Also, people who didn't feel refreshed after a night's sleep had a 27% increased risk of a heart attack, compared with those who did, the researchers added.
In their study, Laugsand's group took into account factors such as age, sex, marital status, education level, blood pressure, cholesterol, diabetes, weight, exercise and shift work, along with depression and anxiety, which can also cause insomnia.
According to the researchers, 33% of the general population has at least one insomnia symptom. In addition, earlier, smaller studies have found a connection between heart disease and insomnia as well as high blood pressure and heart attack.
There are two important limitations to the study, the researchers noted. First, they did not take into account for obstructive sleep apnea, the disorder known to cause disrupted sleep due to pauses in breathing or shallow breathing during sleep. Second, the results may not apply to Americans because daylight hours and sleep patterns are different than those in Norway.
Dr. Gregg Fonarow, a professor of cardiology at the University of California, Los Angeles and a spokesman for the American Heart Association, said a number of earlier studies have investigated whether disturbances of sleep are independently associated with a higher risk of a heart attack.
"These prior studies have yielded mixed results, and it remains unknown if better sleep means a healthier heart," he said.
This new, population-based study found that men and women who reported having difficulties in falling asleep, staying asleep or having restorative sleep had a moderate increase in risk of heart attack over the next decade, Fonarow said.
"Further research is needed to confirm these findings, explore the potential mechanisms involved, and determine if interventions that effectively treat insomnia can reduce the risk of acute myocardial infarction," he added.
One possible explanation for the findings is that all metabolic processes in the body are governed by what are called circadian rhythms, which vary significantly between sleep-awake cycles, said Dr. Edward A. Fisher, The Leon H. Charney Professor of Cardiovascular Medicine at NYU Langone Medical Center in New York City.
"It is known that animals with disrupted circadian rhythms develop metabolic changes that, if they occurred in people, would increase heart disease risk," Fisher said.
"Overall, independent of the exact mechanism, the association shown seems plausible, and is yet another reason to do as the authors advise -- seek professional help for better sleep," Fisher said. "Besides improving the general quality of life, it might even provide cardiovascular benefits."
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